Pumping devices have been developed over the years for extracting oil and gas from wells drilled into subsurface formations. One commonly utilized apparatus is a walking beam pump that includes a sucker rod extending into the ground for thousands of feet and being connected to the walking beam. The walking beam typically driven by an electric motor coupled to the opposite end of the walking beam by a pitman arm to induce a rocking motion into the walking beam to draw oil and/or gas out of the well. The walking beam apparatus has well known and well documented disadvantages and alternative drive mechanisms to move the sucker rod have been developed, such as a pump jack which is low profile and can be mounted above or below ground level together with an adjustable length stroke and extremely low power requirements.
One such alternative drive mechanism is a dual cylinder lift pump as disclosed in U.S. Pat. No. 7,490,674, granted to Marion Brecheisen on Feb. 17, 2009. The pair of hydraulic cylinders driving the movement of the sucker rod in U.S. Pat. No. 7,490,674 had a significant problem in the turn-around phase in changing the direction of movement of the cylinder rod, which in turn changes the direction of movement of the sucker rod. The switching of the open loop spool valve controlling the direction of movement of the hydraulic cylinder results in an abrupt switching of direction, which in turn causes substantial shock, rod stress and rod stretch. With sucker rods extending 5000 to 6000 feet into ground, the sucker rod mechanism can weigh 8000 pounds, which provides a significant resistance to an abrupt change in direction at the hydraulic cylinder driving mechanism.
A continuation-in-part solution to the above-identified problems was disclosed in U.S. Pat. No. 7,600,563, issued to Marion Brecheisen on Oct. 13, 2009, wherein a delay or hesitation cylinder was incorporated into the drive apparatus to resolve this problem, but from a practical application failed to increase reliability and durability.
Another problem associated with an abrupt change in direction for the hydraulic cylinder drive mechanism of an oil well sucker rod, particularly at the top of the stroke of the cylinder rod, is that the hydraulic cylinder tends to “free fall” until the flow of hydraulic fluid from the hydraulic pump can catch up with the movement of the piston. This “free-fall” in practice has been observed to extend for a drop of 12 to 16 inches before stabilizing. This “free-fall” problem is believed to be the result of the use of a spool valve to control the direction of movement of hydraulic fluid through the hydraulic cylinder, as the forces associated with the dropping of the heavy sucker rod cause a faster flow of hydraulic fluid from the cylinder into the reservoir tank faster than the hydraulic pump can fill the opposite side of the piston with hydraulic fluid from the reservoir.
The hydraulic drive mechanism disclosed in the aforementioned Brecheisen patents included a flow control in an attempt to control the speed of operation of the hydraulic cylinders. While the flow control did work, a flow control generates heat, which in a continuous operation like a pump jack drive mechanism results in oil temperatures getting too high. The limit switches in this disclosed drive mechanism were somewhat unreliable, especially in inclement winter weather conditions. The Brecheisen hydraulic cylinder configuration included a hydraulic delivery pipe attached to an external port and extending to the internal fluid passage to deliver hydraulic fluid thereto. Fabrication required extremely close tolerance and sometimes leaked. Ultimately the fitting for the external port had to be a custom made part.
Since these hydraulic pump jack cylinders are long and slender by nature, and are usually transported in a horizontal orientation, these hydraulic pump jack cylinders suffered seal problems. The looseness of the internal hydraulic delivery tube and the seal problems may be attributed to the assembly bouncing internally during transportation. The Brecheisen hydraulic cylinder interior stationary tube is attached only to the alignment ring, which must hold up roughly 10 feet of heavy wall steel tubing, plus the piston assembly, the heavy cylinder head and the inner piston tube.
It would be desirable to provide a hydraulic pump jack cylinder and associated drive mechanism for operating the sucker rod of an oil well that would resolve the aforementioned problems associated with the known pump jack cylinders and provide a sucker rod drive mechanism that would have greater durability and reliability.